In this study positron emission tomography (PET) scanning will be used to explore endogenous opioid system physiology in vivo, in the addictions. Using the positron-emitting ligand Cyclofoxy (6-deoxy-6-b18 fluoronaltrexone), which binds to opioid receptors, the density, availability and distribution of these will be studied. Further, related studies will be performed using 18F-2-fluoro-2-deoxy-glucose (FDG) PET scanning to explore potential changes in glucose metabolism in specific brain regions.